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AP Physics C: Electricity and Magnetism Practice Quiz: Kirchhoff's Junction Rule

Written by AP Content Team, Verified for 2026 AP Exams, Last updated: May 2026

Test your understanding with short quizzes. This quiz has 9 questions to check your progress.

Question 1 of 9

Kirchhoff's Junction Rule is a direct consequence of which fundamental physical principle?

All Questions (9)

Kirchhoff's Junction Rule is a direct consequence of which fundamental physical principle?

A) Conservation of energy

B) Conservation of momentum

C) Conservation of electric charge

D) Conservation of mass

Correct Answer: C

The provided content explicitly states, "Kirchhoff's junction rule is a consequence of the conservation of electric charge."

At a specific junction in an electrical circuit, two currents, 3 A and 4 A, are flowing into the junction. If only one current is flowing out of the junction, what is its magnitude?

A) 1 A

B) 3.5 A

C) 7 A

D) 12 A

Correct Answer: C

According to Kirchhoff's Junction Rule, the total current entering must equal the total current exiting (ΣI_in = ΣI_out). The total incoming current is 3 A + 4 A = 7 A. Therefore, the single outgoing current must be 7 A.

A current of 10 A enters a junction and splits into two outgoing currents. If one of the outgoing currents is 6 A, what is the magnitude of the other outgoing current?

A) 16 A

B) 10 A

C) 6 A

D) 4 A

Correct Answer: D

The rule states ΣI_in = ΣI_out. The total incoming current is 10 A. The total outgoing current is the sum of the two exiting currents, 6 A + I_out2. Therefore, 10 A = 6 A + I_out2. Solving for I_out2 gives 4 A.

Which of the following statements best describes Kirchhoff's Junction Rule?

A) The total voltage around a closed loop must be zero.

B) The total amount of charge entering a junction per unit time must equal the total amount of charge exiting that junction per unit time.

C) The resistance of a junction is always negligible.

D) The electric field inside a conductor at a junction is zero.

Correct Answer: B

This is the definition provided in the content: "Kirchhoff's junction rule states that the total amount of charge entering a junction per unit time must equal the total amount of charge exiting that junction per unit time."

Consider a junction where currents I₁ and I₂ are entering, and currents I₃ and I₄ are exiting. Which equation correctly applies Kirchhoff's Junction Rule to this scenario?

A) I₁ + I₃ = I₂ + I₄

B) I₁ + I₂ = I₃ + I₄

C) I₁ - I₂ - I₃ - I₄ = 0

D) I₁ × I₂ = I₃ × I₄

Correct Answer: B

The rule is expressed as ΣI_in = ΣI_out. In this case, the sum of the incoming currents is I₁ + I₂, and the sum of the outgoing currents is I₃ + I₄. Therefore, the correct equation is I₁ + I₂ = I₃ + I₄.

If an experiment were to measure more total charge exiting a junction per unit time than entering it, what fundamental law would this observation violate?

A) Ohm's Law

B) Newton's Second Law

C) The law of conservation of energy

D) The law of conservation of electric charge

Correct Answer: D

The provided text states that Kirchhoff's Junction Rule is a consequence of the conservation of electric charge. A violation of the rule, where incoming and outgoing charge per unit time are not equal, would mean that charge is either being created or destroyed at the junction, which violates the law of conservation of electric charge.

According to the provided content, what is the primary application of Kirchhoff's Junction Rule?

A) To calculate the power dissipated by a circuit element.

B) To determine the equivalent resistance of a complex circuit.

C) To describe a circuit or elements of a circuit.

D) To measure the voltage drop across a junction.

Correct Answer: C

The first point of the provided content states that the rule is used to "Describe a circuit or elements of a circuit by applying Kirchhoff's junction rule." The other options are not mentioned in the provided text.

At a junction, a 5 A current and an 8 A current are flowing out. A single unknown current, Iₓ, is flowing in. What is the value of Iₓ?

A) 3 A

B) 5 A

C) 8 A

D) 13 A

Correct Answer: D

Using the equation ΣI_in = ΣI_out, the total incoming current is Iₓ and the total outgoing current is 5 A + 8 A = 13 A. For the rule to hold, Iₓ must be equal to 13 A.

Three currents meet at a junction. I₁ = 6 A is entering the junction, and I₂ = 9 A is exiting the junction. What is the magnitude and direction of the third current, I₃?

A) 3 A, entering the junction

B) 3 A, exiting the junction

C) 15 A, entering the junction

D) 15 A, exiting the junction

Correct Answer: A

To satisfy the rule ΣI_in = ΣI_out, the total current in must equal the total current out. Currently, 6 A is entering and 9 A is exiting. To balance this, an additional 3 A must enter the junction. Therefore, ΣI_in = 6 A + 3 A = 9 A, which equals ΣI_out = 9 A.